Electrical apparatus



Patented Nov. 21, 1944 .QVUNIITEDI STATES PATENT OFFICE ELECTRICAL APPARATUS David-Arthur Bell, London, England, assignor to Radio Patents Corporation, a corporation of NewTork Application February 4, 1942, Serial No. 429,509

In Gre'atBritain February 18, 1941 1 which must be sufficiently broadly tuned to re- 11 Claims.

This invention relates to limiters for removing amplitude variations from angular velocity-modulated carrier waves, this term being generic for frequency or phase modulated waves. The invention substantially comprises a method of. limiting the amplitude of a frequency or phase modulated signal wave which consists in heterodyning said wave in a linear rectifier with an auxiliary oscillation having a constant frequency different from the frequency of said signal wave and having a constant amplitude less than the smallis to be limited, and in extracting one of thebeat frequency waves from the heterodyning product of said signal wave and auxiliary oscillation. The invention may be applied in varioustypes of apparatus handling frequency or phase modulated waves, for example, radio transmitters, radio receivers and signal generators. When the invention is applied to radio receivers, the major 'part of the amplificationis normally carried out before the limiting heterodyne stage, so that the amplitude-of the heterodyning oscillation, as applied to the linear rectifier, need not be unduly small. The amplification may be effected at radio frequency. If however the radio receiver is intended for reception of signals at various radio carrier frequencies, it is preferred that thefrequency be changed and, the amplification effected with a fixed intermediate carrier frequency, so that the frequency of the limiting heterodyne oscillator need not be-varied.

In the accompanying drawing, Figure 1 is a circuit diagramof a radio receiver embodying the invention. Figure 1a is a graph illustrative of the function of the invention. Figures 2 and 3 are fragmentary diagrams showing alternative: arrangements for certain stagesof a radio receiver of the kind shown in Figure 1 Figure 4 is a circuit'diagram of a signal generator for generating frequency modulated signals. v

In' the circuit shown in Figure 1, the frequency modulated radio signals picked up by aerial I are developed across aerial tuning circuit 2 and are then applied to first detector 3, in which they are heterodyned with oscillations of suitable fre-- quency generated in the first local oscillator 4. The frequency of this local oscillator is varied est amplitude of said signal wave and to which'it spond to all frequencies within the range which is covered by the frequency modulation of the first intermediate frequency.

Asecond local oscillator I generates oscillations of'constant frequency and amplitude which, by

means of coupling coils 8 and 9, are mixed with the signals developed across tuned circuit 6 an applied to dio'de rectifier I0. I

The amplitude of the oscillation from local oscillator I, as applied to diode I0, must be'smaller than the smallest amplitude of signal from intermediate frequency amplifier 5, as applied to diode II), which it is desired to handle in the receiver. It will then follow that the resultantvoltage at difference frequencydeveloped across tuned circuit I I will be substantially constant.

This function of the invention will be further understood from the following: Let it be assumed that the input of a linear rectifier consists of the sum of the following two oscillations or signal Waves with the tuning of aerial tuning circuit 2 in the conventional manner to maintain a fixed difference beat frequency. The signals at this difference beat frequency are selected and amplified in the first intermediate frequency amplifier 5 I The signal output from intermediate frequency amplifier .5 is impressed across tuned circuit 6,

A sin um: and B sin wit of which the first may represent the incoming signal and the second a locally generated oscillation in accordance with the invention.

I The sum of these two oscillations applied to a linear rectifier-may be written as a single oscillation of complex form as follows:

wherein we and r can be expressed in terms of A, B, an and 202 (see e. g. Terman Radio Engineering, McGraw-Hill Book Company, 1937 edition, pages 448 and 449.

, The amplitude of this complex wave is which may be expanded into shown above need be considered. Accordingly therefore, the amplitude of the resultant wave extracted by the tuned circuit following the rectifier will be determined as follows:

This function rapidly approaches the limit B as A becomes large compared with B as indicated by the graph shown in Figure la which represents From the foregoing it follows that in the ar rangement according to the invention, when the,

amplitude of the input signal is large compared with the amplitude of the local oscillation, the output is determined by the amplitude of the local oscillation and is independent of the amplitude of the input signal. Thus, by properly choosing the amplitude of the local oscillation and maintaining it substantially constant, the input signal may be limited to any desired amplitude such as for the elimination of undesired or spurious amplitude modulation of afrequency modulated si nal wave. The frequency or phase of the output is equal to w1w2 and it therefore faithfully represents the variations in frequency or phase of the input signal. 9

The difference beat frequency signals are selected and amplified in tuned circuit II and the second intermediate frequency amplifier I2.

Tuned circuit I I must be sufficiently broadly tuned to respond; to all frequencies within the range which is covered by the frequency modulation of the second. intermediate frequency. Also, tuned circuit II is so designed as to by-pass signals of first intermediate frequency by a series resonance.

The signal output from intermediate frequency amplifier I2 is applied to discriminator I3 and the resulting amplitude modulated signals are demodulated in demodulator I4 and amplified .in audio-frequency amplifier I5 and finally applied to reproducer I6.

It isdesirable to arrange that the second inter.- mediate carrier frequency, to which circuits II and I2 are tuned, shall be substantially different from the first intermediate carrier frequency, to which circuits 5 and 6 are tuned, so that the passage of signals of first intermediate frequency to the demodulator is minimized. The second intermediate frequency may, however, be selected either higher or lower than the first intermediate frequency.

The parameters of the circuit of rectifier II] can easily be so selected that the constancy of the voltage at second intermediate frequency impressed upon the input terminals of amplifier I2 is not disturbed, even by very sudden variations of the amplitude of the signals received at aerial I.

In the modified arrangement shown in Figure 2, the tuned circuit II and the second intermediate frequency amplifier I2 of Figure l are omitted, and the signals of second intermediate frequency resulting from heterodyning in rectifier ID are directly impressed across the frequency selective circuits of the discriminator. The discriminator diagrammatically shown is of a conventional type comprising circuits I], I8 resonant at frequencies above and below the limits of the frequency modulation range; the amplitude-modulated signals j developed across circuits I'I, I8 being separately demodulated by rectifiers I9, 29 developed across rectifier loads 2I and 22, and added in opposition before application to audio-frequency amplifier I5.

Condensers 37, 38 are series-resonant with the equivalent inductances represented by tuned circuits I'I, I8 at the first intermediate frequency, to by-pass components of that frequency which would otherwise produce some response in the discriminator. These condensers are of low reactance at the second intermediate frequency, which is here assumed to be higher than the first. They are by-passed for direct current by resistances 39,. 49 of high value relative to the condenser reactances at first intermediate frequency.

In the modified arrangement shown in Figure 3, in order to minimize the passage of signals of first intermediate frequency to the demodulator, a balanced heterodyning stage is provided between the output of the first intermediate frequency amplifier 5 and the beat frequency resonant circuit II.

In this arrangement, the output from the first intermediate frequency amplifier 5 is impressed across coil 23, which is coupled to coils 24 and 25. Coil 9, which is coupled to coil 8 introducing the oscillations from local oscillator I, is connected between coils 24 and 25, and its centre'point is earthed through resistance 30. Thus both the locally generated oscillations andthe first intermediate frequency signals are-introduced into the circuit of diode rectifiers 28, 29 in the form oscillations balanced about earth.

Condensers 26, 21 are provided to tune inductances 24, 9, 2 5 to resonance at the first intermediate frequency, the tuning being sufficiently bread for response at all frequencies within the range of frequency modulation thereof.

Linear rectification of the mixed intermediate frequency signals and local oscillations is provided by diode rectifiers 28, 29, and the difference beat frequency output is developed across tuned circuit II for input to the second intermediate frequency amplifier. Alternatively, it could be applied directly to a discriminator as described with reference to Figure 2 Resistance 39 carries the direct component of the rectified current, and provides the necessary rectifier bias. Inductance-s 2'4, 9', 25', however, offer high impedance t0 frequencies higher than the first intermediate frequency. An earth connection is, therefore, provided at the midpoint .of condenser arm 26, 21 for these components.

Inthe signal generator circuit shown in Figure 4, signals of modulated frequency, but not necessarily constant amplitude, generated in oscillator 3I, are amplified in amplifier 32 and impressed across tuned circuit 6, which must be sufficiently broadly tuned to respond to all'frequencies within therange of modulation. Loc'al'oscillator 1 generates oscillations of constant frequency and amplitude, which by means of coupling coils 8 and. 9 are mixed with the signals developed'acrosstuned circuit 9 and applied" to diode rectifier I ll. The amplitude of the oscillation from local oscillator I, as appliedto diode II], will always be smaller than the amplitude of the oscillation from oscillator 3 I as applied to diode ID.

The amplitude of the resultant voltage'at sum or difference frequency developed across tuned circuit II is substantially determined by the amplitude of the oscillation from local'oscillator I; asapplied to diode I6. Continuously variable adjustment of the output voltage. may, therefore, be

amplitude of the diiferenceibeatfre effected by variation of .Lthe position of I the tapr aconstant amplitude which is less than the smallping on potentiometerr33 through'whichthe oscil- --lations from; oscillator are; passed; and J the tapping may be arranged fora-operation by a contr ol knob to provide adjustmentof the amplitude 1 Radio; frequency voltmeter :34 will indicate the 'quency signals developed-across tuned'circuit ll. 1 r

, ..Theioutputvoltagezmaybe varied in large steps ;;rectifiers,"means for impressing the energy to be by adjustment of the moveable contact on attenuator 35. The output is takenbetween this moveable contact and earth at terminals 36.

v I' claim: i g

1. The method of limiting the amplitude an auxiliary oscillation of constant frequency different from the frequency of said wave and of constant amplitude which is smaller than the smallest amplitude to which, said wave is to be limited, and in extracting one of the beat frequency Waves from the heterodyning product of said wave and auxiliary oscillation.

2. The method of limiting the amplitude of an angular velocity-modulated electric signal wave whichcomprises the steps of combining said wave in a linear rectifier with an auxiliary wave of different frequency and having a constant amplitude which is less than the smallest amplitude to which said signal wave is to be limited, and extracting difference frequency energy from the heterodyning product of said waves.

of an 1 angular velocity-modulated wave which consists in heterodyning the wave in a linear rectifier with I amplitude to which said signal energy is to be p estamplitu'de to which said signal wave energy is .to. be": limited and means for extracting energy from said output circuit having a frequency equal to the difference-between the frequency of said first and second energies. I

7. .An amplitude limiter for angular velocitymodulated electric signal wave energy comprising output 'circu'its and. including a pair of linear limited upon one of said input circuits, further means for producing and impressing auxiliary wave energy uponthe other input circuit of said modulator, said auxiliary energy having a constant'frequency different from said signal energy, further means for'adjusting the amplitude of said auxiliaryenergy to a valve less than the smallest limited, and means for extracting energy from said signal output having a frequency equal to the difference between thefrequencies of said signal 3. The method of limiting the amplitude of an.

angular velocity-modulated electric signal wave which comprises the steps of combining said wave in a linear rectifier with an auxiliary wave having a frequency different from said signal wave, adjusting the amplitude of said auxiliary Wave to a value less than the smallest "amplitude to which said signal wave is to be limited, and extracting difference frequency energy from the heterodyning product of said Waves.

4. 'An amplitude limiter for angular velocitymodulated electric signal wave energy comprising a linear rectifier, means for applying to said rectifier the wave energy to be limited, further means for applying to said rectifier auxiliary wave energy of a frequency different from said signal wave energy and having a constant amplitude I less than the smallestamplitude to which said signal energy is to be'limited, and means for exand auxiliary energies. I

8. An amplitude limiter for angular velocitymodulated electric signalwave energy comprising means for producing auxiliary wave energy of constant frequency different from the frequency of said signal energy and having an amplitude which is less than the smallest amplitude to which said signal energy is to be limited, means including linear rectifying means for intermodulating said signal energy with said auxiliary energy, and further means for extracting from the intermodulation product energy having a frequency equal to the difference between the frequencies of said signal and auxiliary energies.

9. An amplitude limiter for angular velocitymodulated electric signal wave energy comprising means for producing auxiliary Wave energy of constant frequency different from the frequency of said signal energy, means for adjusting the amplitude of said auxiliary wave energy to a value less than the smallest amplitude to which said signal energy is to be limited, means including tra'cting difference frequency energy from the output of said rectifier.

5. An amplitude limiter for angular velocitymodulated electric signal wave energy comprising a linear rectifier, means for applying to said rectifor applying to said rectifier auxiliary .Wave energy of afrequency different from said signal wave the smallest amplitude to which said signal wave fier the wave energy to be limited, further means linear rectifying means for intermodulating said signal energy with said auxiliary energy, and means for extracting from the intermodulation product energy having a frequency equal to the differencebetween the frequencies of said signal and auxiliary energies. v

10. In a system for receiving angular velocitymodulated electric waves of varying carrier frequency, means for changing the frequency of a a'balan'ced' modulator having a pair of input and received signal Wave and amplifying it at a first F fixed intermediate carrier frequency, amplitude limiting means comprising a linear rectifier, means for applying to said rectifier the amplified intermediate frequency wave and an unmodulated heterodyning oscillation having a constant frequency different from said intermediate frequency and having an amplitude which is less than the smallest amplitude to which said intermediate energy is to be limited, and means for extracting difference frequency energy from the output energy of said rectifier. v v

6. An amplitude limiterfor angular velocitymodulated electric signal Wave energy comprising a balanced modulator circuit having a pair of input and an output circuits-and including a pair of linear rectifiers, means for impressing the energy to be limited upon one of said input circuits, further means for producing and impressing auxiliary wave energy upon the other input circuit, said auxiliary energy having a frequency different from said signal wave energy and having frequency wave is to be limited, means to extract from the output,of said rectifier energy at the difference beat" frequency resulting from said oscillation and the amplified intermediate fre..

,quency wave, and means for demodulating and limited angu- *lar velocity-modulated wave energy.

translating the resultant amplitude 11. In a system for receiving angular velocitymodulated electric waves of varying carrier frequency, first heterodyning means for changing the frequency of a received signal wave and amplifying it at a firstfixed intermediate carrier frequency, amplitude limiting means comprising secintermediate frequency to a. :seccnd intermediate frequency :and including linear rectifying means,

means for applying tosaid rectifying means the amplified intermediatefrequency wave andran unmodulated heterodyning,.oscillationhaving a. constant frequency different from :sa'idintennediate frequency and having an amplitude which :is less than the smallest amplitude to which said inter-- mediate frequency Wave is t be limited, means to extract from the output of said rectifying means and heterodyning means for changing .said first second intermediate frequency energy at the difference beat frequency resulting fromz'said oscilletioniand the amplified first intermediate frequency wave, -means for demcduletingnnd trans- .lating the resultant intermediate frequency .a mplitude limited angular velocity-modulated wave energy, and means for :preventingienergy of the first intermediate-frequency from interfering with the second intermediate .ifrequency energy.

' LDAVID BELL. 

